Catalytic processes for the conversion of hydrocarbons are well known and extensively used. Invariably the catalysts used in these processes become deactivated for one or more reasons. Where the accumulation of coke deposits causes the deactivation, reconditioning of the catalyst to remove coke deposits restores the activity of the catalyst. Coke is normally removed from catalyst by contact of the coke containing catalyst at high temperature with an oxygen-containing gas to combust and remove the coke in a regeneration process. These processes can be carried out in-situ or the catalyst may be removed from a vessel in which the hydrocarbon conversion takes place and transported to a separate regeneration zone for coke removal. Arrangements for continuously or semi-continuously removing catalyst particles from a reaction zone and for coke removal in a regeneration zone are well known.
In order to combust coke in a typical regeneration zone, a recycle gas is continuously circulated to the combustion section and a flue gas containing by-products of a coke combustion, oxygen, and water is continually withdrawn. Coke combustion is controlled by recycling a low oxygen concentration gas into contact with the coke-containing catalyst particles. The flue gas/recycle gas is continuously circulated through the catalyst particles. A small stream of makeup gas is added to the recycle gas to replace oxygen consumed in the combustion of coke and a small amount of flue gas is vented off to allow for the addition of the makeup gas. The steady addition of makeup gas and the venting of flue gas establishes a steady state condition that produces a nearly constant concentration of water and oxygen as well as the combustion products in the recycle gas.
In continuous or semi-continuous regeneration process, coke laden particles are at least periodically added and withdrawn from a bed of catalyst in which the coke is combusted. Regions of intense burning that extend through portions of the catalyst bed develop as the coke is combusted. After this intense burning the catalyst requires reconditioning to restore and re-disperse the noble metal, usually platinum, to its most highly catalytic state. Reconditioning for a dehydrogenation catalyst will include contact with a chloride containing compound, to redistribute the noble metal and replace the chloride that may be lost from the catalyst, followed by a drying step to reduce the moisture content of the catalyst and finally a reducing step to change the noble metal from various oxidized states to a reduced metallic condition. A preferred noble metal is platinum.
A number of environmental and operational problems have been associated with these catalyst reconditioning steps. Replacing chloride on the catalyst and re-dispersing platinum over the surface of the catalyst are both done in the presence of a chloride compound. However, the catalyst platinum re-dispersion benefits from a high chlorine environment whereas chloriding is usually effected more efficiently in the presence of hydrogen chloride. The chlorine and hydrogen chloride in the chloride contact zone are in equilibrium with the water and oxygen present therein. This equilibrium is skewed towards higher concentrations of hydrogen chloride. In order to provide adequate chlorine for re-dispersion of the platinum metal, the total concentration of hydrogen chloride must be relatively high. Maintaining the high hydrogen chloride environment adds to the expense of designing the regeneration zone by requiring the recycle of hydrogen chloride-containing gas. In most cases, this recycle of the gas is done by a closed loop system, a heater, a blower and associated piping. The expense of this equipment is compounded by the fact that exotic materials are needed to withstand a hydrogen chloride environment. Moreover, gas must be vented from the halogenation loop that circulates the hydrogen chloride containing gas. This vented gas has a high concentration of hydrogen chloride and must, therefore, be vented or treated in a way that avoids damage to equipment or the environment. Another drawback associated with the high hydrogen chloride environment is that there is often more hydrogen chloride uptake on the catalyst than is necessary or desired.
This invention provides a process for the dehydrogenation of hydrocarbons which incorporates a method of reactivating a noble metal catalyst that has been deactivated by the accumulation of coke on its surface during dehydrogenation and requires regeneration to remove coke and needs re-dispersion of the noble metal to provide adequate catalytic activity. This invention is particularly suited for catalysts that use platinum metals and maintain a chloride concentration on the catalyst particles. In such cases, the arrangement and operation of this process will improve the re-dispersion of platinum on the catalyst particles and allow a better control of the chloride content on the reconditioned catalyst particles. This invention can also reduce emissions and handling problems associated with hydrogen chloride containing gases and can reduce the overall expense of operating a dehydrogenation process with the concomitant reconditioning of catalyst particles used therein.